Homework Help Forum: Math 350..help!

Posted by Marilyn on Tuesday, October 20, 2009 at 1:43am.

How would I start this? The amount that workers contribute monthly for health insurance premiums can be modeled by A(t)=0.07t^3-3.1t^2+54.3t-230, where A is the monthly amount contributed and t is the number of years after 1980. Find the instaneous rate of change in monthly contribution in 2000.

• Math 350..help! - jim, Tuesday, October 20, 2009 at 7:38am

  Don't let all the digits and decimals throw you off. The number of years after 1980 just means that in 1981, t = 1, and 1982, t = 2, and so on. You need to differentiate 0.07t^3 - 3.1t^2 + 54.3t - 230 Which, really, is just differentiating t^3, t^2, and t and multiplying them by their respective constants. Then find the valuw of the derivative function at t=20. Is that enough of a start?

  
  

• Math 350..help! - Marilyn, Tuesday, October 20, 2009 at 10:26am

  I'm using the formula for instantaneous rate of change, but I'm getting -38.4 as my answer..what am I doing wrong? 560-1240+1086-230-560-1240+1086-230/20 -2480+2172-460/20

  
  

• Math 350..help! - bobpursley, Tuesday, October 20, 2009 at 10:35am

  A(t)=0.07t^3-3.1t^2+54.3t-230 A'=.21t^2-6.2t+ 54.3 Now you are a t=20 so A'=84-12.4+54.3 How did you get -38?

  
  

• Math 350..help! - Marilyn, Tuesday, October 20, 2009 at 12:40pm

  If the question says use the formal definition of a derivative aren't I supposed to use f(x)=f(x+h)-f(x)/h?

  
  

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